The organic food market made up $29 billion in U.S. sales in 2012—a figure that’s certainly not insignificant but still looks like pocket change next to the $479 billion in sales Walmart did in the last fiscal year

Organic farmers have an issue with scale—although there are some examples of organic yields meeting conventional yields, in general, organic production just can’t produce as much food per unit of land as regular farming. As such, organic food costs more to make. If Walmart is going to start selling it without a markup, those shortfalls are going to hit someone.

Over recent decades, rising demand for organic foods has already been squeezing American farmers, says NPR. Unable to keep up with demand, American farmers have turned to importing organic grains from Asia. Working under the scale and stability of a Walmart contract could give the company’s partners confidence to invest locally. But, until that can happen, the environmental and global health costs of increased international shipping ultimately detract from an organic farm’s potential benefits. This adds to the fact that, though organic farms are better for the environment on a “per acre” basis, they’re actually worse on a “per product” basis.

Aside from all that, the mass adoption of organic agriculture isn’t a practical plan from a more fundamental standpoint. As it stands, organic farms rely on nutrients trickling down from conventional farms. Many organic products rely, indirectly, on chemical fertilizers and pesticides. As organic agriculture gains a larger share of food production then, some scientists suggest, crop yields could drop even further.

We overestimate the sustainability of organic and underestimate the sustainability gains of conventional farming in less than helpful ways. The sooner we move away from defining sustainable agriculture by ideology and towards measuring it by metrics, the better.